Check out my snapshot. I have got Cherry Audio's GX80 inserted on the Instrument track at the top.

With the audio track directly underneath, left click on the Inputs, triangle that points downward. A menu now appears similar to the one shown in my picture. Because I have Instrument and Audio tracks present they both show up. Click on the Instrument track and select the name of the synth you have got inserted on it. In my case it is GX80.

The Instrument track synth now becomes the input to the audio track. If you have got other audio tracks present, notice you can also select those as input sources.

And by the way you can render anything that is on an Instrument track. Without doing any of this. Right click on the Instrument track header. A menu now appears as per my second shot. Scroll down to Transform to Audio Track. Click on that and you will get some options. This will transform the Instrument track to Audio and keep the midi data behind intact. You can also go back anytime you want as well.